platform/x86: ISST: Optimize CPU to PCI device mapping

It was observed that some of the high performance benchmarks are spending
more time in kernel depending on which CPU package they are executing.
The difference is significant and benchmark scores varies more than 10%.
These benchmarks adjust class of service to improve thread performance
which run in parallel. This class of service change causes access to
MMIO region of Intel Speed Select PCI devices depending on the CPU
package they are executing.

This mapping from CPU to PCI device instance uses a standard Linux PCI
interface "pci_get_domain_bus_and_slot()". This function does a linear
search to get to a PCI device. Since these platforms have 100+ PCI
devices, this search can be expensive in fast path for benchmarks.

Since the device and function of PCI device is fixed for Intel
Speed Select PCI devices, the CPU to PCI device information can be cached
at the same time when bus number for the CPU is read. In this way during
runtime the cached information can be used. This improves performance
of these benchmarks significantly.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Link: https://lore.kernel.org/r/20210616221329.1909276-1-srinivas.pandruvada@linux.intel.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

Author: spandruvada

drivers/platform/x86/intel_speed_select_if/isst_if_common.c

@@ -281,11 +281,27 @@ static int isst_if_get_platform_info(void __user *argp)
 struct isst_if_cpu_info {
 	/* For BUS 0 and BUS 1 only, which we need for PUNIT interface */
 	int bus_info[2];
+	struct pci_dev *pci_dev[2];
 	int punit_cpu_id;
 };
 
 static struct isst_if_cpu_info *isst_cpu_info;
 
+static struct pci_dev *_isst_if_get_pci_dev(int cpu, int bus_no, int dev, int fn)
+{
+	int bus_number;
+
+	if (bus_no < 0 || bus_no > 1 || cpu < 0 || cpu >= nr_cpu_ids ||
+	    cpu >= num_possible_cpus())
+		return NULL;
+
+	bus_number = isst_cpu_info[cpu].bus_info[bus_no];
+	if (bus_number < 0)
+		return NULL;
+
+	return pci_get_domain_bus_and_slot(0, bus_number, PCI_DEVFN(dev, fn));
+}
+
 /**
  * isst_if_get_pci_dev() - Get the PCI device instance for a CPU
  * @cpu: Logical CPU number.
@@ -300,17 +316,18 @@ static struct isst_if_cpu_info *isst_cpu_info;
  */
 struct pci_dev *isst_if_get_pci_dev(int cpu, int bus_no, int dev, int fn)
 {
-	int bus_number;
+	struct pci_dev *pci_dev;
 
 	if (bus_no < 0 || bus_no > 1 || cpu < 0 || cpu >= nr_cpu_ids ||
 	    cpu >= num_possible_cpus())
 		return NULL;
 
-	bus_number = isst_cpu_info[cpu].bus_info[bus_no];
-	if (bus_number < 0)
-		return NULL;
+	pci_dev = isst_cpu_info[cpu].pci_dev[bus_no];
 
-	return pci_get_domain_bus_and_slot(0, bus_number, PCI_DEVFN(dev, fn));
+	if (pci_dev && pci_dev->devfn == PCI_DEVFN(dev, fn))
+		return pci_dev;
+
+	return _isst_if_get_pci_dev(cpu, bus_no, dev, fn);
 }
 EXPORT_SYMBOL_GPL(isst_if_get_pci_dev);
 
@@ -327,6 +344,8 @@ static int isst_if_cpu_online(unsigned int cpu)
 	} else {
 		isst_cpu_info[cpu].bus_info[0] = data & 0xff;
 		isst_cpu_info[cpu].bus_info[1] = (data >> 8) & 0xff;
+		isst_cpu_info[cpu].pci_dev[0] = _isst_if_get_pci_dev(cpu, 0, 0, 1);
+		isst_cpu_info[cpu].pci_dev[1] = _isst_if_get_pci_dev(cpu, 1, 30, 1);
 	}
 
 	ret = rdmsrl_safe(MSR_THREAD_ID_INFO, &data);